1. Field of the Invention
The present invention relates to an ink-jet printing apparatus and a recovery treatment method in the ink-jet printing apparatus mainly for stabilizing color reproduction ability of an output image.
2. Description of the Related Art
As the conventional ink-jet printing apparatus, there is so-called serial scan type ink-jet printing apparatus exchangeably mounting a printing head as printing means and ink tanks as ink containers on a carriage movable in a primary scanning direction. This printing system sequentially performs printing on printing mediums by repeating primary scan of the carriage mounting the printing head and the ink tank and auxiliary scan (feeding) of the printing mediums.
Considering realizing a micro-printer applicable for PDAs (Personal Digital Assistants) or s cameras, since the size of the carriage must be small, the storage capacity of ink containers to be mounted on the carriage has to be extremely small. If storage capacity of the ink tank on the carriage is extremely small, frequency of exchange of the ink tanks would become high, and exchange of the ink tanks during a single printing operation would become necessary.
In order to solve the problem, Japanese Patent Application Laid-Open No. 2000-334982 discloses an ink-jet printing apparatus employing an ink supply system, in which each time when the carriage is moved to a predetermined stand-by position, ink is supplied from a separately provided ink receptacle member (hereinafter referred to as a main tank which is normally of much greater volume than the ink tank on the carriage) to the ink tank on the carriage (hereinafter referred to as a sub-tank) at a given appropriate timing (also referred to as pit-in ink supply method).
In this apparatus, at every occasion of printing an image on one printing medium for example, the carriage has to be moved to the predetermined stand-by position and the sub-tank and the main tank are connected with each other by a joint member at an appropriate timing for filling the ink from the main tank to the sub-tank. Accordingly, the problem due to quite small ink storage capacity of the sub-tank on the carriage can be solved.
However, in the construction set forth above, the inventors have gotten the following finding as a result of extensive study. When the ink-jet printing apparatus is left in a non-use state for a relatively long period and thereafter used for printing, the color tone of the image could become unnatural. Also, when the same image is printed for a number of times, color tones between images of a plurality of sheets could be different.
Such unnatural color tone or inconsistency of color between the printed products of the same image is particularly not favorable as a printer for cameras for printing photographs.
Such a phenomenon is caused due to condensation of the ink in the sub-tank by leaving the printing apparatus in a low humidity environment for a long period of time. This problem can be reduced by providing a mechanism closing an opening portion of the sub-tank as required, selecting material of the sub-tank to the one having smaller gas permeability or increasing thickness of the sub-tank.
However, these measures cannot be ultimate solutions unless evaporation becomes zero. Also, such measures could cause an increase of costs and enlargement of sizes of the sub-tanks to hinder down-sizing.
On the other hand, according to further extensive study made by the inventors, it has been found that when the ink-jet printing apparatus is left in the non-use state for a relatively long period of time, viscosity of the ink in the sub-tank is significant to reach the ink viscosity far beyond the ink viscosity of the ink normally used in the ink-jet printer to make it impossible to recover nozzles of the printing head.
FIGS. 19A to 19D are schematic representations for explaining a relationship between the sub-tank and remaining amount of ink in the sub-tank in time series. At first, FIG. 19A shows a state where ink is filled in the sub-tank in a pit-in ink supply system. When printing is completed, a state is reached where the ink amount used for printing is consumed, as shown in FIG. 19B. It should be noted that, in the case of application of the pit-in ink supply system to a compact printer, the sub-tank has a quite small capacity. For example, ink storage amount per color is 0.4 ml (=400 μl). In FIG. 19A, 0.4 ml of ink is filled. In FIG. 19B, 0.2 ml, which is half of ink filled in the sub-tank, is consumed and 0.2 ml of ink remains.
As left in the state shown in FIG. 19B, volatile components, such as water, in the ink are evaporated from the sub-tank. While the evaporation speed of the volatile components is variable depending upon material and thickness of the sub-tank, and material, structure and so on of the cap for preventing ink in the nozzle of the printing head from drying, the volatile components are nevertheless evaporated at a certain rate. For example, assuming that the evaporation speed in each color of ink is 0.002 ml per day (=2 μl/day), about 100 μl is evaporated in fifty days, and an evaporation rate from the initial weight becomes 50%. After being left for an even longer period, while the evaporation speed can be lowered slightly, it finally reaches a state where the volatile solvent components in the ink are completely evaporated (state shown in FIG. 19C). It should be noted that the evaporation rate or speed referred to herein is the evaporation rate under conditions where drying is most significant among operation guaranteed environmental conditions.
As an ink composition to be used in the typical ink-jet printing apparatus, a coloring component such as non-volatile dye or pigment is less than or equal to about 10%, the amount of solvent having low volatility (e.g. glycerin, ethylene glycols) is about 15% to 40%, and the remaining contents are volatile water or alcohols. Strictly, the solvent having low volatility evaporates in a little amount. However, since the evaporation amount of such solvent having low volatility is far smaller than that of water or the like, such coloring component and solvent having low volatility is hereinafter referred to as “non-volatile solvent” for the purpose of explanation, and the ratio is assumed to be 25%. Then, in the foregoing example, the ink remaining amount 200 μl×volatile component ratio 0.75=150 μl can be evaporated. Assuming that 2 μl is evaporated per day, the volatile component such as water can be completely evaporated in about seventy-five days. This point will be referred to as the evaporation limit (in practice, further evaporation is continued even after the evaporation limit since the solvent having low volatility evaporates a little amount gradually).
While depending upon the composition of the ink, the viscosity of such ink is about 2.0 mPas in a non-evaporated state and 10.0 mPas in a 50% evaporated state in a case of the ink in the sixth embodiment of the present invention, which will be discussed later. In contrast to this, the viscosity of the ink evaporated up to a 75% of evaporation limit reaches greater than or equal to about 400 mPas, which is greater than or equal to about two hundreds times the ink viscosity in a normal, non-evaporated state.
When such ink of high viscosity is present in the nozzle, ink cannot be sucked by a suction recovery method of the conventional ink-jet printing apparatus, whereby ejection failure can be caused in the nozzle. It should be appreciated that such phenomenon is a problem specifically found in the pit-in ink supply system using the sub-tank of small capacity, in which condensation of ink becomes high over time, thereby leaving a small amount of ink in the sub-tank.